Using the audio plug in portable electronic devices, such as smartphones or tablets, for data transmission to and from external devices is commonly known. For example, patent application US2013087614 discloses magnetic stripe readers that communicate data via an audio interface for receiving data from a magnetic card reader. In US2013087614 filtering is performed at the card reader side.
However, audio interfaces, typically 3.5 mm female stereo/microphone sockets and corresponding hardware and software, in portable electronic devices are generally optimized for the transfer of voice communication and not for data transformed to audio signals. The audio interfaces may comprise hardware or software for optimizing the sound experience or quality. However, when using audio interfaces for other purposes than audio, such as for data communication, this optimization may cause a problem, because this optimization makes the data difficult to interpret.
Today, one way to encode data of this kind is to use Manchester Encoding. When using Manchester encoding the data signal changes from a low level to a high level in the middle of the bit period when a binary “0” is transmitted (changing input “0” to “01”) and, conversely, changes from a high level to a low level in the middle of the bit period when a binary “1” is transmitted (changing input “1” to “10”). In other words, it is a coding format that divides a single bit period into a front cell and a rear cell, expressing a logical value “0” when the front cell is at a low level and the rear cell is at a high level, and expressing a logical value “1” when the front cell is at a high level and the rear cell is at a low level. It can also be said that the Manchester encoding converts a single input bit into two bits (or transmits a single bit in two pulses (2 T)), with the communication rate being halved by doubling the bandwidth, but the direct current component being eliminated from the transmitted signal.
FIG. 4 shows how a signal may be distorted by the optimizations of the audio interface. The upper signal 40a shown in FIG. 4 discloses a Manchester coded signal transmitted from a card reader device. The middle signal 40b shows the received signal, i.e. the raw data of the sound buffer after being processed by the hardware and software of the audio interface. In this example the distortion is so severe that a traditional decoder may fail to decode the signal correctly.
Hence, a great number of portable electronic device models are rendered non-compatible to receive data such as Manchester encoded data on the audio interface, due to failure to decode the data, since the audio interface comprises e.g. amplifiers, filters, A/D converters and other software and hardware components, which are optimized for audio or voice communication.